1. Technical Field of the Invention
The present invention relates to a harmonic speed reducer for robots, which is used for a joint or other members of robots including robots used in factories.
2. Related Art
It has been known that multijoint robots, for example, have a configuration in which a harmonic speed reducer is used in actuating and rotating each joint by a motor. The harmonic speed reducer is based on a harmonic Drive® technique. Such a harmonic speed reducer has an advantage of having a large speed reducing ratio and thus is very useful as a speed reducer for robots.
FIG. 1 shows an example of a configuration of a harmonic speed reducer in a robot. A harmonic speed reducer 81 includes as main components a housing 82, a frame 83, a circular spline (internal tooth gear) 84, a flexspline (external tooth gear) 85, and a wave generator 86. The frame 83, which is hollow, is formed in the housing 82 of a robot arm. The circular spline 84 having an annular shape is provided with inner teeth at its inner periphery and fixed to the frame 83. The flexspline 85 having a cylindrical shape is provided with outer teeth at its outer periphery for engagement with the inner teeth of the circular spline 84. The wave generator 86 having an elliptical shape is fitted to the flexspline 85 along its inner periphery.
The wave generator 86 is connected to a rotary shaft, i.e. an input shaft, of a servomotor 87. The flexspline 85 is connected to an inner race 88a, i.e. an output shaft, of a cross roller bearing 88. The inner race 88a, or the output shaft, of the cross roller bearing 88 is connected to an arm or the like.
The circular spline 84 is mounted on a circular spline mounting seat 83a of the frame 83 with the aid of a circular spline holder 89 (hereinafter also just referred to as “holder 89”) and screws 90. In this regard, the seat 83a is formed with internal threads 91, while the circular spline 84 is formed with screw insertion holes 92 corresponding to the internal threads 91. The holder 89 as well is formed with screw la insertion holes 93 corresponding to the internal threads 91. The screws 90 are inserted into the respective insertion holes 93 of the holder 89 and the respective insertion holes 92 of the circular spline 84 for threadable engagement with the respective internal threads 91. The circular spline 84 is mounted on the seat 83a in this way.
FIG. 2 shows a procedure of forming the seat 83a and each internal thread 91. As shown in FIG. 2, a circular spline mounting seat is formed by machining in a coarse cast member that constitutes a robot housing (step a1). Then, a pilot hole for internal thread is drilled (step a2). Finally, the pilot hole is tapped to form an internal thread (step a3).
Recently, robots are requested to have a higher degree of repeat accuracy (how many times the same trajectory can be repeatedly drawn). The repeat accuracy tends to be adversely affected by vibration having irregularly changing intensity. The cause of vibration in a robot is ascribed to the vibration of harmonic speed reducers used in the robot. Therefore, it is necessary to mitigate the vibration of each of the harmonic speed reducers in order to achieve a high degree of repeat accuracy of the robot.
For example, a patent document JP-A-H11-264448 discloses a method of mitigating vibration of a harmonic speed reducer used for robots. According to this patent document, errors caused by vibration are measured for each robot. Then, an error value calculated from the measurement is incorporated in the control of the robot as a correction value. As a result, a control error that would have been caused by the vibration is ensured to be eliminated for the improvement of the repeat accuracy.
When the method of the conventional art mentioned above is used in mass production, the method necessitates such processes as measurement of vibrations of each robot, calculation of an error value based on the measured vibrations, and preparation of the error value as a correction value. It is true that the repeat accuracy can be improved through these processes but the production efficiency will be extremely lowered. Therefore, these processes are not suitable for mass production. A different approach may be to apply error value of a certain robot to all of other robots in order to raise the production efficiency. This however will not lead to the improvement of the repeat is accuracy, because the errors caused by vibration depend on each specific robot.